FIG. 1 is a schematic diagram of a MIG welder. A welding "gun" 3 carries a welding electrode 6 which takes the form of a flexible wire. When an operator (not shown) brings the wire 6 into contact with the object 9 to be welded, an electric arc 12 jumps from the wire 6 to the object 9, fusing them both, and causing a weld puddle (not separately shown) to form. When the wire 6 is withdrawn from the object, the arc is broken, and the weld puddle solidifies.
The wire 6 is consumed during the welding, and so feeder wheels 15, driven by a motor 18, continuously supply additional wire from a storage reel 21. When the object to be welded is a metal that is subject to rapid oxidation by atmospheric oxygen at the high temperatures created by the arc (such as aluminum), an inert gas supply 23 delivered to a nozzle 24 is used to shroud the arc and weld puddle in a cloud 25 of inert gas. The inert gas prevents the oxidation.
In the prior art, electric power for a welder is typically supplied by a transformer 27 as shown in FIG. 2. The secondary winding 30 of the transformer 27 delivers power to the welding electrode, indicated as resistive load 33, typically through Silicon Controlled Rectifiers labeled SCR. In operation, the SCRs trigger alternately, one after the other, thus alternately conducting current, as indicated by arrows 36. A control circuit 39 controls the timing of the firing of the SCRs, thus controlling their duty cycle and the power delivered to the load 33.
One characteristic of the system just described is that the SCRs tend to be expensive because they must have a high current-carrying capacity. For example, a single SCR rated at 200 amps can cost substantially more than four SCRs having a 50-amp rating each.